为什么Python和wc对字节数不一致？

Question

Python并且wc对给定字符串的字节数(长度)有很大的不同意见:

with open("commedia.pfc", "w") as f:
    t = ''.join(chr(int(b, base=2)) for b in chunks(compressed, 8))
    print(len(t))
    f.write(t)

Output : 318885

---

$> wc commedia.pfc
  2181  12282 461491 commedia.pfc

该文件主要由不可读的字符组成,因此我将提供一个hexdump:

http://www.filedropper.com/dump_2

该文件是无前缀压缩的结果,如果您要求我可以提供生成它的完整代码以及输入文本.

为什么两个字节数都不相等？

---

我添加了压缩算法的完整代码,它看起来很长但是充满了文档和测试,所以应该很容易理解:

"""
Implementation of prefix-free compression and decompression.
"""
import doctest
from itertools import islice
from collections import Counter
import random
import json

def binary_strings(s):
    """
    Given an initial list of binary strings `s`,
    yield all binary strings ending in one of `s` strings.

    >>> take(9, binary_strings(["010", "111"]))
    ['010', '111', '0010', '1010', '0111', '1111', '00010', '10010', '01010']
    """
    yield from s
    while True:
        s = [b + x for x in s for b in "01"]
        yield from s

def take(n, iterable):
    """
    Return first n items of the iterable as a list.
    """
    return list(islice(iterable, n))

def chunks(xs, n, pad='0'):
    """
    Yield successive n-sized chunks from xs.
    """
    for i in range(0, len(xs), n):
        yield xs[i:i + n]

def reverse_dict(dictionary):
    """
    >>> sorted(reverse_dict({1:"a",2:"b"}).items())
    [('a', 1), ('b', 2)]
    """
    return {value : key for key, value in dictionary.items()}

def prefix_free(generator):
    """
    Given a `generator`, yield all the items from it
    that do not start with any preceding element.

    >>> take(6, prefix_free(binary_strings(["00", "01"])))
    ['00', '01', '100', '101', '1100', '1101']
    """
    seen = []
    for x in generator:
        if not any(x.startswith(i) for i in seen):
            yield x
            seen.append(x)

def build_translation_dict(text, starting_binary_codes=["000", "100","111"]):
    """
    Builds a dict for `prefix_free_compression` where
       More common char -> More short binary strings
    This is compression as the shorter binary strings will be seen more times than
    the long ones.

    Univocity in decoding is given by the binary_strings being prefix free.

    >>> sorted(build_translation_dict("aaaaa bbbb ccc dd e", ["01", "11"]).items())
    [(' ', '001'), ('a', '01'), ('b', '11'), ('c', '101'), ('d', '0001'), ('e', '1001')]
    """
    binaries = sorted(list(take(len(set(text)), prefix_free(binary_strings(starting_binary_codes)))), key=len)
    frequencies = Counter(text)
    # char value tiebreaker to avoid non-determinism                     v
    alphabet = sorted(list(set(text)), key=(lambda ch: (frequencies[ch], ch)), reverse=True)
    return dict(zip(alphabet, binaries))

def prefix_free_compression(text, starting_binary_codes=["000", "100","111"]):
    """
    Implements `prefix_free_compression`, simply uses the dict
    made with `build_translation_dict`.

    Returns a tuple (compressed_message, tranlation_dict) as the dict is needed
    for decompression.

    >>> prefix_free_compression("aaaaa bbbb ccc dd e", ["01", "11"])[0]
    '010101010100111111111001101101101001000100010011001'
    """
    translate = build_translation_dict(text, starting_binary_codes)
    # print(translate)
    return ''.join(translate[i] for i in text), translate

def prefix_free_decompression(compressed, translation_dict):
    """
    Decompresses a prefix free `compressed` message in the form of a string
    composed only of '0' and '1'.

    Being the binary codes prefix free,
    the decompression is allowed to take the earliest match it finds.

    >>> message, d = prefix_free_compression("aaaaa bbbb ccc dd e", ["01", "11"])
    >>> message
    '010101010100111111111001101101101001000100010011001'
    >>> sorted(d.items())
    [(' ', '001'), ('a', '01'), ('b', '11'), ('c', '101'), ('d', '0001'), ('e', '1001')]
    >>> ''.join(prefix_free_decompression(message, d))
    'aaaaa bbbb ccc dd e'
    """
    decoding_translate = reverse_dict(translation_dict)
    # print(decoding_translate)
    word = ''
    for bit in compressed:
        # print(word, "-", bit)
        if word in decoding_translate:
            yield decoding_translate[word]
            word = ''
        word += bit
    yield decoding_translate[word]


if __name__ == "__main__":
    doctest.testmod()
    with open("commedia.txt") as f:
        text = f.read()
    compressed, d = prefix_free_compression(text)
    with open("commedia.pfc", "w") as f:
        t = ''.join(chr(int(b, base=2)) for b in chunks(compressed, 8))
        print(len(t))
        f.write(t)
    with open("commedia.pfcd", "w") as f:
        f.write(json.dumps(d))
    # dividing by 8 goes from bit length to byte length
    print("Compressed / uncompressed ratio is {}".format((len(compressed)//8) / len(text)))
    original = ''.join(prefix_free_decompression(compressed, d))
    assert original == text

commedia.txt 是filedropper.com/commedia

Answer 1

您正在使用Python3和str对象-这意味着你在看到数len(t)是多少个字符的的字符串中.现在,字符不是字节 - 从90年代开始就是如此.

由于您没有声明显式文本编码,因此文件编写使用系统默认编码对文本进行编码 - 在Linux或Mac OS X上将为utf-8 - 一种编码,其中任何字符都不在ASCII范围内( ord(ch)> 127)在磁盘上使用多个字节.

所以,你的程序基本上是错误的.首先,定义您是处理文本 还是字节.如果您要处理字节,请打开文件以便以二进制模式写入(wb而不是w)并更改此行:

t = ''.join(chr(int(b, base=2)) for b in chunks(compressed, 8))

至

t = bytes((int(b, base=2) for b in chunks(compressed, 8))

很明显,你正在使用字节本身,而不是破坏字符和字节.

当然,有一个丑陋的解决方法是对字节对象执行文本的"透明编码" - (如果您的原始列表将包含0-256范围内的所有字符代码点,那么):您可以编码之前的在将其写入文件之前t使用latin1编码.但这在语义上是错误的.

您还可以尝试Python的鲜为人知的"bytearray"对象:它使人们能够处理8位数字的元素,并且具有可变和可扩展的便利性(就像具有足够内存空间的C"字符串"一样)预先分配)